The invention pertains to smoke sensors of a type used in fire detectors. More particularly, the invention pertains to such sensors having a reduced size and a low profile.
Fire or smoke detectors have become widely used elements of fire alarm systems. Such alarm systems often incorporate large numbers of such detectors spread over substantial regions to detect and track the build-up of smoke.
Known detectors while effective for their purpose have at times been regarded as less than aesthetically pleasing due to their profile and over-all size. There thus continues to be an on-going need for smaller detectors having lower profiles and a smaller over-all size.
While small chamber size has been recognized as being preferable from an aesthetic and architectural point of view, it has also been recognized that as chambers become smaller the signal to noise, ratio can potentially drop and become less than optimal. As chamber dimensions have become smaller, background light levels detected in photoelectric smoke chambers by the respective light sensitive element (such as a photodiode or a phototransistor) can increase significantly. There continues to be a need for smoke sensors which while physically small exhibit appropriate signal to noise ratios while minimizing nuisance alarms.
A photoelectric sensing chamber has a cylindrical shape with a relatively low profile. A base element is formed with a cylindrical region and a closed end. A cover has a hollow cylinder which extends therefrom. The cover slideably engages the base such that the distal end of the cylinder is located adjacent to the base. Together they form a substantially enclosed, cylindrical symmetrical sensing chamber. The chamber encloses a symmetrical sensing region.
The cover carries a plurality of openings at an exterior, proximal, end displaced from the distal end of the cylinder. The openings permit ingress and egress of adjacent ambient atmosphere, which could carry smoke or particles of combustion.
An annular flow path extends between the base and the cylinder, coupled to the openings. This path, around the cylinder and extending to the base couples the openings to the sensing region.
The cylinder cooperates with the base to form an inflow/outflow region between the annular flow path outside of the cylinder and the internal sensing region. This produces a more or less U-shaped flow path which is symmetrical around the sensing region.
The symmetrical flow path and symmetrical internal sensing region are achieved by displacing a source of radiant energy, such as a light emitting diode or laser diode and a sensor of scattered radiant energy, such as a photodiode or a phototransistor, into the base of the chamber outside of the internal sensing region. With this configuration, the shape of the source does not distort and detract from the symmetry of the sensing region. Similarly, by displacing the sensor into the base, its shape does not distort the symmetrical shape of the sensing region.
Each of the source and the sensor can be, in one aspect of the invention, located in conduits displaced from the sensing region. One conduit, in addition to supporting the source, provides a focusing function for the radiant energy being projected into the sensing region. Another provides a collecting function for scattered incident light directed to the sensor. This increases optical gain of the chamber.
In another aspect of the invention, protrusions can be provided in the conduit for the sensor to block a first reflection of light from the source off of the internal side wall of the sensing chamber to provide an enhanced signal to noise ratio. Such protrusions for example could occupy 20 to 40 percent of the area of the respective conduit to produce the noise suppressing function. A preferred percentage is on the order of 27 percent.
A protrusion in the conduit for the source cooperates with the interior geometry of the conduit to block and reflect a portion of the light injected through the conduit by the source. This also contributes to the enhancement of the signal to noise ratio.
The conduits are located at an angle relative to one another which corresponds to the primary scattering angle for the sensing chamber. In this regard, for laser sources, an angle can be established in a range of 20 to 30 degrees. A 25 degree angle is preferable. For infrared light emitting diodes, an angle can be established in a range of 40-45xc2x0.
In another aspect of the invention, the orientation of the conduits directs the beam of light from the source and directs the field of view of the light sensitive element toward opposite sides of the grooved interior surface of the chamber. The source projects a spot of radiant energy, or light, onto the opposite wall of the sensing chamber, the internal grooved side wall of the cylinder. Preferably in this embodiment, no light will illuminate the fringe of the cover cylinder. However, if due to component variations, emitted radiant energy illuminates the cover fringe, the above-noted protrusion in the conduit for the sensor should block any resultant stray light from reaching the sensor.
The opposite side of the cover cylinder, which is intersected by the optical axis of the sensor does not receive any direct illumination from the source. As such, the sensor is directed to a region having low levels of stray background light or radiant energy.
Hence, the orientation of the conduits taken together reduces the degree of stray background light or radiant energy which can find its way onto or into the light sensor. This in turn contributes to an enhanced signal to noise ratio and a detectable level of scattered light in response to smoke permeating the sensing region.
In another aspect of the invention, the inner surfaces of the side wall and the bottom of the chamber can be formed with grooves to promote absorption of light and to provide depressed regions for accumulating dust that has drifted into the sensing chamber.
In yet another aspect of the invention, the cylinder which extends from the cover has a continuous closed peripheral surface without perforations therethrough. Ambient atmosphere including ambient smoke, flows up and down the continuous side walls to and from the sensing region. Consequently, the cover, in yet another aspect of the invention, can incorporate a screen or a mesh at an exterior end thereof. Mesh openings can have a length in a range of 0.013xe2x80x3 to 0.02xe2x80x3 long.
The mesh can be inserted into the mold before the cover/cylinder are molded. Alternately, the openings can be molded into the cover without a separate mesh or screen.
The nested cylinders, namely the cylinder carried on the cover and the cylinder formed by the base provide a substantially continuous annular flow path into the sensing region unlike known multiple vane labyrinths which result in several, restricted flow paths into the sensing region. A substantially continuous opening around the exterior perimeter of the cover of the housing can be provided for ingress and egress of smoke.
Taking into account the above-noted characteristics and features, results in a sensing chamber height on the order of 0.7 inches or less with a diameter of less then 1.5 inches. This produces a sensing volume of less than 1.24 cubic inches and an optical spacing on the order of 1.35 inches.
The smaller sensing volume reduces time to respond to incoming ambient smoke. Additionally, a smaller mesh size can be used, thereby improving exclusion of insects and dust, while at the same time, the chamber still exhibits an acceptably short response time to ambient smoke.
Increasing the size of the mesh or screening of the chamber will also shorten response time. Thus, sensing chambers in accordance with the invention produce increased signal to noise ratios as a result of a combination of reduced sensing region volume, and appropriately selected screen or mesh size in combination with the symmetry of the sensing region and the protrusions in the optical conduits which reduce background chamber noise.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.